Thermionic cathode lamp and method



Jan. 23, 1934. A, w L Re. 19,057

THERMIONIC CATHODE LAMP AND METHOD OF OPERATING THE SAME Original FiledOct. 15. 1927 Inventor: Albert W. Hull,

H is Attorney.

Reissued Jan. 23, 1934 UNITED STATES THERMIGNIC OATHODE LAlVlP ANDMETHOD OF OPERATING THE SAME Albert w. Hull, Schenectady, N. Y,assig'nor to General Electric Company, a corporation of New YorkOriginal No. 1,929,143,

dated October 3, 1933,

Serial No. 228,275, October 15, 1927. Application for reissue November27, 1933. Serial No.

18 Claims.

The present invention relates to gaseous glow lamps, that is, to lampsin which an electric dis charge is conducted for illuminating purposesthrough a column of suitable gas or vapor, such, for example, as neon ormercury vapor.

In accordance with my invention 1 have produced a new form of gaseousglow lamp having a high efliciency, long life and other advantages andwhich is provided with a thermionic cathode.

Glow lamps heretofore have been provided with non-thermionic electrodes,that is, electrodes which are caused to emit electrons by bombardmentbut which are not operated during use at a temperature 01' thermalelectron emission. As the electron emissivity of such non-thermionicelectrodes is relatively small per unit of area, it has been necessaryfor practical purposes to operate columnar lamps with currents wellbelow 100 milliamperes. Even at this relatively low current value, itwas necessary to provide such a lamp with awkwardly large electrodes.

In such gaseous glow lamps having non-thermionic or cold" electrodesthere is a relatively large voltage drop at the cathode which representsa loss of energy, and which causes disintegration or sputtering oi thecathode and blackening of the lamp walls. In the case of neon lamps thisvoltage drop is about 200 volts, or more. In order that this largevoltage drop should not constitute too large a fraction of the totaloperating voltage and thus render the operating eiiiciency of such lampsuncommercially low, it was necessary to construct such lamps foroperation at voltages of several thousand volts.

The intrinsic brilliancy of the illumination of such low current lampsbeing correspondingly low, their usefulness was limited. For example,they could not be used most effectively for display or advertisingpurposes in places where the general illumination is high, the contrastwhich makes such monochromatic illumination effective being partiallylost under these conditions.

It was suggested heretofore to substitute a thermionic filamentarycathode in place of the non-thermionic cathodes in gaseous glow lamps,but the lamps proposed were not suitable ior operation with substantialcurrents nor for long periods and the filamentary cathodes were subjectto excessive electrical disintegration or sputtering.

I have discovered that thermionic cathode gaseous glow lamps can beconstructed to give a thousand hour or longer period 01' operation withcurrents higher than 100 milliamperes (1/10 ampere), if the cathode tallor potential is maintalned below a limiting value which I have termedthe disintegration voltage. This may be done by so correlating theelectron emissivity of the cath ode with relation to the space or glowcurrent which the lamp is designed to carry that the fall of potentialat the cathode does not exceed the disintegration voltage. Thedisintegration voltage" is always greater than the ionization voltage ofthe gas content of a lamp and in a lamp having a luminous column ofappreciable length is materially less than the difference of potentialbetween the electrodes.

In operation at current values 01' high luminous efficiency a commerciallength of lamp life has not heretofore been obtained in gaseous glowlamps employing thermionic cathodes. In thermionic gaseous glow lampsheretofore proposed the cathode has been a filament. It such a cathodeis to give an adequate electron emission the filament must either be 01'such length that the potential drop between its terminals is greaterthan the disintegration voltage, or of increased thickness, in whichcase the magnetic held of the heating current interferes with orprevents an eflective electron emission. I1 more than one filament wereoperated in parallel the discharge would run from onefllament only andcause its rapid destruction.

While my invention in a general sense includes gaseous glow lamps havingcathodes of any form or shape providing their electron emissivity issufliciently high to support a desired operating current, withoutcausing the fall of voltage at the cathode to be greater than the"disintegration voltage, in the preferred embodiments of my invention,indirectly heated non-filamentary cathodes are employed constructed tofurnish electron emission capable of supplying a spacecurrent which thelamp is designed to carry at a temperature of inappreciable thermaldisintegration and which operate either with no potential drop betweenextremities or operate with potential drop less than the disintegrationvoltage. Such electrodes, for example, may assume the term 01'relatively thin walled hollow bodies coated with a thermionically activematerial and being maintained at operating temperature by radiation orconduction 01 heat from a suitable heater. In an a nuated gas,electrodes coated with a thermionically active material can be operatedat a higher temperature than in a vac uum without loss of the coatingmaterial but the foundation material slowly evaporates at the mosteffective operating temperature. Unlike filamentary electrodes which areheated by the application of current at their terminals, electrodes suchas herein shown and described will continue to function even after thenickel, or other foundation material, has been dissipated to such extentthat only a skeleton or lace-work remains. Also, unlike lamps containingfilamentary cathodes, no tendency exits in lamps embodying my inventionfor the discharge to concentrate at the negative terminals. Todistinguish such broad area cathodes over filamentary cathodes whichburn out quickly in a glow lamp due to local disintegration, I shallrefer to cathodes having such effective configuration as broad cathodes.

Lamps embodying my invention having anilluminating column of moderatelength and containing a gas or vapor of high luminosity, such as neon ormercury vapor, may be operated on ordinary commercial lighting circuitsof 110 to 115 volts, with a commercially long life, at lightingefiiciencies formerly attainable only in non-thermionic glow deviceswhen operating at voltages of four to five thousand volts. Lampsembodying my invention may be operated at current values materiallyabout l/ 10 ampere and ordinarily of one to several amperes with highintrinsic brilliancy, and a luminous efiiciency of about 15 lumens perwatt. They may be constructed in large units giving a higher totalillumination than was possible heretofore. The discharge is diffuse,filling the entire discharge space, but otherwise is arc-like in itscharacteristics.

My invention which includes a new method of lamp operation as well as anew device will be explained in greater detail in connection with theaccompanying drawing of which Fig. 1 is a side elevation of a preferredembodiment of my invention containing a permanent gas; Fig. 2illustrates an operating circuit for alternating current and also amodification in which mercury vapor is used; Fig. 3 illustrates a directcurrent operating circuit and also shows in side elevation anothermodification in which the enlargement of the envelope about the cathodeis absent, and Figs. 4 and 5 are enlarged sectional views of cathodestructures.

Referring to Fig. 1, the lamp here shown embodying my invention has atubular envelope 1 consisting of glass, silica, or other suitabletransparent material. This envelope has been shown as broken to indicatethat it has a considerable and varied length depending in any particularcase on conditions. For example, for a 110 volt lamp, containing neon anenvelope 2.5 cm. in diameter and about to cm. long may be used. Theanode 2 carried by' a stem 3, which is sealed into the press, .4, is ofthe usual construction, and may consist of nickel, iron, molybdenum,their alloys, or other suitable material. The cathode 5 may beconstituted of a relatively thin cylinder of nickel coated with asuitable thermionically active material. I may use, for example, analkaline-earth compound, such as barium carbonate, preferably with asuitable binder, such as a solution of a cellulose compound. Thiscathode is heat treated to render it active. This treatment consists inraising the temperature of the coated cathode to about 1050 to 1300" C.while the envelope 1 is being evacuated of gas. In the case of afoundation of nickel coated with barium, the forming treatmentpreferably should be carried close to the melting point of nickel.Evacuation is continued during this heat treatment in order to removegaseous decomposition products. The barium carbonate is decomposed and acompound or alloy of barium and nickel is formed which has a metallicappearance as contrasted with the white appearance of the bariumcarbonate coating. During the operation of the lamp the cathode shouldbe heated to a temperature somewhat below the formation temperature ofthe coating, say, to a temperature of about 1000 C.

The cathode is provided with an internal heater 6 consisting oftungsten, or other suitable material, receiving current by theconductors'l, 8, which are sealed into the press 9, the conductor 7being joined to one end of the heater 6, and the conductor 8 beingjoined to the closed end of the cylinder 5. The latter in turn is joinedelectrically to the opposite end of the heater. The cathode structure iscarried by a stem 10 fused into the glass press 9. The envelope 1 isexpanded about the cathode into the form of a bulbous cathode chamber11, to avoid overheating. It is charged after thorough evacuation with asuitable gas, such as neon, for example, at a pressure of, say aboutseveral microns to several m.m. of mercury. As shown in Fig. 2. a dropof mercury 12 may be introduced as a source of vapor. The length of thedischarge path should be great enough to cause the fall of potential inthe luminous column tobe greater than the combined fall of potential atthe electrodes, and ordinarily should be chosen to cause this potentialdrop to be several times greater than the combined electrode potentialdrop. A tube having a diameter of about 2.5 cm. containing neon at apressure of about two millimeters of mercury has a voltage drop of aboutone volt per cm. with a current of about three amperes. At lowercurrents the voltage drop is greater as at this pressure the volt-amperecharacteristic is slightly negative. This voltage drop varies inverselywith the tube diameter. The potential drop is substantially constantover a pressure range from about two to five millimeters. The lightintensity varies substantially as the 0.67 power of the current. As thevoltage drop across the lamp decreases with increase in current, theluminosity is roughly proportional to the wattage consumption. A neonlamp embodying my invention operating with a drop of potential of about120 volts in the lamp at a current of 2.9 amperes, and an energyconsumption at the cathode of about 34 watts, gave a light emission ofabout 5350 lumens, that is, an efficiency of the discharge of aboutfifteen lumens per watt, and an overall efficiency of 10.7 lumens perwatt, that is, taking into account losses in the stabilizing resistanceand other losses.

A lamp embodying my invention may be constructed for operation eitherwith direct or alternating current. I have illustrated in Fig. 2 a lamphaving two anodes 14, 15, located in the branches 16, 17 of the vitreousenvelope, these anodes being connected to the opposite terminals of thesecondary of a transformer 18. The cathode 19 is connected by theconductor 20 to an intermediate point of the secondary of thetransformer 18. An auxiliary transformer 21 is shown for heating thecathode although a battery or other suitable heating means may be used.The conductor 20 is connected in series with the coil of a contactor 22to an intermediate point of the secondary of the transformer 18, thatis, to approximately the neutral point.

A high frequency device, such as illustrated at 23 may be employed tofacilitate starting of the discharge, not only of the device shown inFig.

2, but in connection with any 01 the lamps embodying my invention. Suchdevice and its action in starting is well known and hence will not behere described. When an operating current 5 flows through the lamp, themagnet oi the contactor 22 is energized, attracting its armature 24 andopening the primary circuit oi the cathode heating transformer 21. Themain current, which now passes through the cathode heater on its way tothe anode maintains the cathode heated to an operating temperature. Thecathode oi the device shown in Fig. 2, as better shown in Fig. 5, isprovided with two heater spirals 25, 25, connected in series with oneanother and also connected at 26 to the cathode shell 19. These heaters25, 25' are imbedded in suitable reiractorwinsulating material 27, iorexample, magnesium oxide, contained within the shell 19. The exterior oithe shell 19 is coated with suit able activating material. Current issupplied by the conductors 28, 23'.

Another modification of my invention is shown in Figs. 3 and 4, claimsbeing made on the cathode construction oi this modification in adivisional application Serial No. 633,143, flied August 1, 1933. Thecathode cylinder 29 is hollow and contains a single heater spiral 30,one end oi which is connected to a supply wire 31, the other end beingconnected to the shell 29. The shell 29 in turn is electricallyconnected by the ring-shaped member 32 to the shield 33. The shield 33conserves heat and coating material which otherwise would be lost byradiation and evaporation irom the cylinder 23 and thereby materiallyincreases the emciency and life oi the cathode. Contact is made to thisshield by a sealed-in conductor 34. A steadying support 35 is providedat the side oi the cathode opposite the point of connection to theconductor 34. The exterior of the shell 29 and the interior of theshield 33 are.coated with a material oi high thermionic emissivity asdescribed above. A spacing and insulating sleeve 36 consisting oialumina, or the like, preterably is provided between the conductor 31and the shell 29. Either a iixed gas or a vapor may be introduced intothe envelope 1, Fig. 3, aiter the gas and moisture content have beenremoved and the cathode has been activated.

In a copending application Serial No. 156,713, filed by me on December23, 1926 claims are made on a gas-containing electrical discharge deviceembodying a thermionic cathode having a cavity coated with a material ofhigh thermionic emissivity. I

The lamp of Fig. 3, which is provided with such a cathode, requires noenlargement about the cathode because oi the shielding eil'ect oi thecylinder 33. The lamp is shown connected by a circuit 3'! to a directcurrent source 38 in series with a resistance 39, a hand operated switch40, and the coil 41 oi a mercury switch 42. The mercury switch 42 is ina parallel circuit 43 containing a resistance 44 and a switch 45. Whenthe switches 40 and 45 are closed, current flows in the heater circuit43, which thereupon is opened by the mercury switch, causing a highvoltage impulse to be impressed upon the lamp. This action of the coil41 may be repeated a number of times until the glow discharge isstarted. If desired a heating current also may be impressed on theconductors 31, 34 to bring the cathode to an operating temperaturepreliminary to starting, this current being interrupted after the lampis started.

When the heater 30 is properly proportioned with respect to the energysource 33, the external resistance 39 may be omitted. The currentthrough the gas in the lamp flowing through the heater 30 maintains thecathode at operating temperature. Although in the preierred iorms of myinvention the cathode heater serves also as a resistance for limitingthe space or glowproducing current to or below the limiting value abovewhich the cathode drop oi potential would exceed the disintegrationvoltage and cause excessive sputtering of the cathode by positive ionbombardment, I wish it to be understood that various known means, suchas an external resistonce or a constant current source may be used forthe same purpose.

In U. S. Patent 1,790,153 flied concurrently herewith broad claims aremade for gas-containing electrical discharge devices which are providedwith thermionic cathodes constructed and proportioned to provide athermionic electron emission suflicient to support in the absence ofpositive ion bombardment the operating current such devices are designedto carry.

What I claim as new and desire to secure by Letters Patent 01' theUnited States, is:

1. An electric lamp comprising a sealed envelope, a luminosity producinggas therein at about one millimeter of mercury pressure, and cooperatingelectrodes including a broad thermionic cathode which is capable ofsupporting an arc discharge in said gas oi at least an ampere, saidelectrodes being separated a distance sumciently great to cause thetotal voltage drop between the same to be several times the ionizationvoltage oi said gas, a source of current and means ior limiting thevalue oi the space current in said device with respect to the electronemission of the cathode to maintain the fall oi voltage at said cathodeless than the cathode disintegration voltage.

2. An electric lamp comprising an elongated, tubular envelope, 2 charge01' neon gas therein at a pressure oi! about one to live millimeters oimercury, electrodes therefor comprising a broad thermionic cathodehaving an electron emissivity of several amperes at about l000 C., anelectric heater for said cathode and means for electrically operatingsaid lamp in series with said heater.

3. An electric glow lamp comprising a lighttransmitting envelope, acharge of gas therein which is capable of producing luminosity andhaving a pressure within the range of a iew microns, to a fewmillimeters oi mercury, electrodes mounted in said envelope, one oi saidelectrodes being coated with a material oi high emissivity, and a shieldspaced closely about said coated electrode.

4. An electric lamp comprising a light transmitting envelope, anelectrically luminous gas therein, and electrodes insaid envelope spacedapart such distance that the iall of potential in the luminous columnbetween said electrodes is greater than the combined potential falls atsaid electrodes, said electrodes including a cathode coated withalkaline-earth material oi high electron emissivity and having an arealarge enough regions spaced apart such distance that in a luminousdischarge between said electrodes the fall of potential in the gasbetween said electrodes will materially preponderate over the fall ofpotential at said electrodes, one of said electrodes,

being a hollow metal body coated with alkaline earth material,and anelectrical resistance heater for said coated electrode, said coatedelectrode having a sufficient area to emit without the aid of positiveion bombardment a current of at least about an ampere at a temperaturegiving a commercially long life.

6. An electric lamp comprising a light-transmitting envelope, a chargeof neon gas therein at a pressure of about several microns to severalmillimeters of mercury, and electrodes mounted in said envelope atregions spaced apart such distance that in a luminous discharge betweensaid electrodes the fall of potential in the gas between said electrodeswill materially preponderate over the fall of potential at saidelectrodes, one of said electrodes being a hollow body of nickel coatedwith alkaline earth material of sufficient area to maintain withoutpositive ion bombardment a current the lamp is designed to carry at atemperature of about 1000 C. and a heater for said coated electrode.

7. An electric lamp comprising a container, a charge ofluminosity-producing gas therein at a pressure within the range of aboutseveral microns to several millimeters, electrodes comprising athermionic cathode and an anode spaced apart a suflicient distance tohave during operation a voltage therebetween which is materially higherthan the disintegration voltage for said gas, said cathode providing ata temperature of inappreciable thermal disintegration a thermionicelectron emission suilicient to supply without the aid of positive ionbombardment substantially the entire operating current which said lampis designed to carry.

8. An electric lamp comprising an envelope, a luminosity-producing gastherein, and electrodes therein spaced apart a sufficient distance topermit of the formation of a luminous positive column therebetweenhaving a fall of potential materially higher than the disintegrationvoltage for said gas, one of said electrodes being a thermionic cathodewhich is capable of supporting an arc discharge of a current value ofabout one to several amperes substantially independently of positive ionbombardment of said cathode, and with a voltage drop at said cathodebelow the disintegration voltage. for said gas.

9. An electric lamp comprising a transparent envelope, a charge ofluminosity-producing gas therein at a pressure within the range of aboutseveral microns to several millimeters, electrodes comprising athermionic cathode and an anode having a fall of voltage therebetweenduring operation which is materially higher than the disintegrationvoltage for said gas, said cathode being constructed and proportioned toprovide at a temperature of inappreciable thermal disintegration athermionic electron emission to supply substantially the entireoperating current which said lamp is designed to carry without the aidof positive ion bombardment of said cathode, and means for limiting theoperating current not to exceed said value.

10. An electric lamp comprising a sealed envelope, a gaseous filingtherefor, electrodes rein one of which comprises a metal shell, 9.material of high electron emissivity coatingsaid body, means for heatingsaid body to a temperature of thermionic emission, said electrode beingcapable at a temperature of inappreciable disintegration of emitting anelectron current of at least about an ampere, and a shield spacedclosely about said body whereby radiation of heat and loss of coatingmaterial are materially reduced.

11. An electric lamp comprising a sealed envelope, an attenuated gastherein, a thermionic cathode therein which is capable at operatingtemperature of emitting an electron current of at least about an ampere,comprising a metal shell, a heater therefor, a material of high electronemissivity coating said shell, and a metal shield closely spaced aroundsaid shell, and an anode cooperating with said cathode.

12. An electric lamp comprising a sealed envelope, a gaseous fillingtherefor at a pressure of about one millimeter of mercury, electrodestherein one of which comprises a hollow metal body, an electric heaterfor. said hollow electrode and means for electrically operating saidlamp in series with said heater.

13. An electric lamp comprising an envelope, a gas therein at a pressurewithin a range of about several microns to several millimeters ofmercury, electrodes therein one of which comprises a hollow metal body,a coating thereon of thermionically active material, an electric heaterenclosed by said hollow electrode, and means for electrically operatingsaid lamp in series with said heater.

14. The method of operating a positive column electric lamp containing ahighly attenuated gas, a thermionic cathode coated with a material ofhigh electron emissivity and a cooperating anode ,which consists inheating said cathode to a temperature of inappreciable thermalevaporation and conducting between said electrodes a current having avalue not in excess of the limiting value at which the fall of potentialat the cathode rises above the disintegration voltage for said gas.

15. The method of operating at voltages of at least about 110 volts apositive column electric lamp containing a gas at a pressure within therange of about several microns to several millimeters of mercury, andbeing provided with a thermionic cathode and an anode, which consists incausing said cathode to emit electrons capable of supporting a designedcurrent at a temperature at which disintegration of said cathode isinappreciable and limiting the current between said electrodes to thedesigned value.

16. The method of operating a positive column electric lamp containing agas at a pressure of about a millimeter of mercury, a. thermioniccathode and an anode spaced apart at least about 50 centimeters and withimpressed voltages materially higher than the cathodic disintegrationvoltage for said gas which consists in causing said cathode to emit bythermionic emission at current of about one to several amperes at atemperature at which thermal disintegration of said cathode isinappreciable and operating between said electrodes a current not inexcess of the limiting value at which the fall of potential 19,057egress of an electrical discharge. a material of high electronemissivity in said enclosure, and

means for heating said cathode to a temperature of thermionicemissivity, said cathode being capable at a temperature of inappreciabledisintegration oi' emitting an electron current of at least about anampere.

18. An electric lamp comprising a container, a charge ofluminosity-producing gastherein, electrodes therefor comprising athermionic cathode and an anode spaced apart a sumcient distance topermit of the formation of a luminous positive column therebetween, saidcathode having an area large enough to provide a thermionic electronemission sumcient to supply a current of at least about an ampere at atemperature at which thermal disintegration is inappreciabl'e, and witha fall of potential at the cathode so low that electrical disintegrationalso is inapprociable.

ALBERT W. HULL.

CERTIFICATE OF CORRECTlGN.

Reissue No. 19,057. January 23, 1934.

ALBERT w. HULL.

ll is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction as follows: Page 2,line 25, for "about" read above; page 3. line 128, claim 3, strike outthe comma after "microns"; page 4, line 72, claim 10, for "filing" readfilling; and that the said Letters Patent should be read with thesecorrections therein that the same may conform to the record of the casein the Patent Office,

' Signed and sealed this 6th day of March, A. .D. 1934.

F. M. Hopkins (Seal) Acting Commissioner of Patents.

